Field of the Invention
The present invention relates to a reference voltage circuit which supplies a reference voltage that can be calibrated by a calibrating device.
Integrated circuits that are not operated from a stabilized supply voltage require a reference voltage source internally. This applies, in particular, to voltage regulators whose output voltage serves as a reference voltage for other integrated circuits or circuit blocks.
In principle, the on-state or forward voltage of a diode or generally of a pn junction, e.g. the base-emitter voltage of a bipolar transistor, can be used as a reference voltage. However, the forward voltage of a pn junction has a negative temperature coefficient, which has an adverse effect for many applications. If, by way of example, it is intended to supply sensors, A/D converters or similar components with the aid of a voltage regulator whose output voltage serves as a reference voltage, the output voltage of the voltage regulator must be highly accurate and, in particular, extremely stable with regard to temperature.
Therefore, band-gap reference voltage circuits are preferably used as reference voltage sources that supply a temperature-stabilized reference voltage. These known band-gap reference voltage sources are based on addition of a forward voltage of a pn junction through which current flows, and a difference voltage multiplied by a corresponding factor, which difference voltage is formed from two voltages of two pn junctions through which different current densities flow. Generally, the forward voltage of a pn junction through which current flows has a negative temperature coefficient as has already been explained above. On the other hand, the difference between two forward voltages rises proportionally to the absolute temperature and is thus subject to a positive temperature coefficient. If the factor by which the difference voltage explained above is multiplied is set in such a way that the negative temperature coefficient of the forward voltage of the pn junction and the positive temperature coefficient of the difference voltage cancel each other out, it is possible to obtain a temperature-stabilized output or reference voltage. In particular, the output voltage of such a reference voltage source, which is obtained by addition of the above-explained forward voltage of a pn junction through which current flows, to the difference voltage (likewise explained above), amounts to approximately 1.25 V, which corresponds approximately to the band-gap of silicon. Therefore, such reference voltage sources are referred to as band-gap reference voltage sources.